This invention relates in general to a communications interface device whereby a facsimile transceiver is enabled to send and receive copies of documents over a two-way voice radio transceiver; and more particularly to a communications interface device which contains electrical interface means which, when connected to a standard facsimile transceiver, simulate a two-wire public switched telephone line; which contains facsimile data modulator and demodulator means for converting facsimile image data into audible tones, and audible tones into facsimile image data; which contains destination addressing means to allow a user to selectively transmit a facsimile message to any one of multiple compatible devices on the same radio channel; which provides error detection and correction means which assure error-free communications over the radio channel; and which contains radio interface means which, connected to a voice radio, simulate an external microphone, talk switch, and speaker.
Facsimile transceivers have commonly been used to send and receive copies of written documents over two-wire voice telephone lines. Such a facsimile transceiver contains station selection (dialing) means to place calls over the public switched voice telephone network; answering means to detect ringing signals from the telephone central office; and electrical coupling means to send and receive analog electrical signals in the voice frequency range, at signal levels and frequencies compatible with the voice telephone network. Such a facsimile transceiver also contains optoelectronic document scanning means, which, for each horizontal scan line of a predetermined height across the document to be copied, produce a string of digital data, where for example a binary one digit represents a black picture element of a predetermined size, and a binary zero digit represents a white picture element. Such a transceiver also contains modulation means which, for transmission, convert digital image data into tones in the voice frequency range, as well as demodulation means which, for reception, convert audio tones into binary image data. Such a facsimile transceiver also contains hard-copy printing means, such as a thermal print mechanism which makes images by selectively heating elements on a thermal print head in contact with thermally-sensitive paper, or a laser print mechanism which produces images on bond paper using a xerographic process, or similar print mechanism; such printing means convert the received digital image data into a printed document, reproducing a copy of the transmitted document. Such facsimile transceiver also includes a timing and control means which control and coordinate the above elements in accordance with procedures and protocols established by telephone authorities and by international telecommunications standards bodies.
There are many applications in which it would be desirable to send and receive documents using such a facsimile transceiver, but where telephone lines are not available. Examples include mobile vehicles, temporary work locations, and remote and undeveloped areas.
Various means have been developed to send and receive digital data over wireless radio links. A radio modem for example may be connected between a computer or data terminal and a radio transceiver. The means for data transfer between the computer or data terminal and the radio modem comprise a direct electrical connection of two closely-located data devices, for example, an RS-232 serial data interface (Electrical Industries Association Standard RS-232). The radio modem contains means for converting digital data received from the computer or data terminal into electrical signals which modulate the carrier of the connected transceiver. Conversely, the radio modem demodulates signals received from the radio so as to regenerate digital data, and passes the data to the connected computer or data terminal. However, a facsimile transceiver cannot be connected to such a radio modem, which lacks the telephone line interface means required for communicating with standard facsimile transceivers.
Other means have been developed to allow devices with telephone-line interfaces--telephone sets, data modems, and the like--to communicate over wireless radio links. A radiotelephone set for example contains telephone line simulation means, coupled to or combined with a radio transceiver. If a facsimile transceiver were connected at each side, two radiotelephones could send and receive messages. Unfortunately, communications errors may frequently be introduced by noise and interference on the radio communications bands, and such a radiotelephone set does not contain means for detecting and correcting such errors. A short burst of noise that may only be annoying in a voice conversation can render unintelligible several lines of image data in a facsimile transceiver connection. Furthermore, a radiotelephone system is generally restricted to only two stations, because neither the radiotelephone set nor the facsimile transceiver contains means for selectively transmitting to one of several stations which use the same communications channel. Still further, some radiotelephones use two radio frequencies, one for each voice direction. In urban and other developed areas, available radio channels are scarce, and licensing authorities are reluctant to assign channels to parties of only two users, the more so where two radio channels are required by each system. These factors limit the usefulness of radiotelephone equipment for facsimile communications.
Means have also been developed recently which allow devices with telephone-line interfaces to communicate over cellular telephone systems. Such a cellular telephone interface device contains a telephone line simulation means, combined with a cellular telephone interface means. If a facsimile transceiver were connected to such a device, it could send and receive messages over the cellular telephone network. Unfortunately, as with radiotelephones, communications errors may frequently be introduced by noise and interference on the radio communications channels. Further, cellular systems periodically switch the radio channel in use from one frequency to another, to improve signal strength for moving cellular telephones, and even with fixed stations to balance channel usage; such channel switching causes brief lapses in communications which are only a minor disturbance for voice conversations, but result in data errors for facsimile equipment. Such a cellular telephone interface device does not provide means for detection and correction of communications errors. Furthermore, cellular telephone service is still unavailable in many remote and undeveloped areas throughout the world. Even where cellular service is available, the use of cellular telephones is economically impractical for many high-volume commercial and industrial users, because of the relatively high prices charged by cellular telephone providers for each minute of service; such users often choose to use independently-owned two-way radio equipment in preference to a cellular telephone system.
It is therefore a primary object of the present invention to provide a communication system which permits a conventional facsimile transceiver to send and receive messages over a two-way voice radio transceiver.
A more specific object of the present invention is to provide a simulated telephone line interface to the facsimile transceiver such that a user may send and receive facsimile messages using operating procedures substantially the same as though the facsimile transceiver were connected to the public switched telephone network.
A further object of the present invention is to provide a modulator and demodulator means which convert audible tones received from the attached facsimile transceiver into digital image data which can be temporarily stored within the device for subsequent transmission over the connected radio, and conversely, which convert stored digital image data previously received from the connected radio into audible tones, so that received messages can be delivered to the connected facsimile transceiver for hard-copy printing.
A further object of the present invention is to provide a means of simulating the circuit switching action of the public telephone network by converting the destination station address "dialed" by the user at the facsimile transceiver into a digital address which is embedded within each block or "packet" of image data subsequently transmitted over the radio, and by further providing that only the addressed station will retain and deliver the message to its attached facsimile transceiver. This allows three or more devices to share the same radio channel.
A further object of the present invention is to provide a error detection and correction means which segment facsimile image data into packets of digital bits prior to transmission over the radio channel; which perform a mathematical operation on the data contained within each such packet to develop a unique cyclical redundancy check value for that packet; which embed this check within each packet at time of transmission; which continue to store each packet, after transmission, until positive acknowledgement of correct reception is received from the destination station; which upon reception of a packet of image data from another station, independently develop the check value for the data actually received, compares it with the transmitted check value, and then causes a response to be returned to the transmitting station acknowledging positively that the packet of data was successfully received, or negatively that an error was detected; which, after transmission of a packet of image data, upon reception of a positive acknowledgement, continue with the following packets of image data, or upon receipt of negative acknowledgement, or if no acknowledgement is received within a specified time, retransmit the packet of data; which contain mechanisms for successively repeating the transmission of image data packets that have not yet been successfully delivered; and which provide sufficient temporary image data storage capacity to hold transmitted and received packets of image data during the process of error detection and correction.
A further object of the present invention is to provide a voice radio interface means which include a modulator that encodes facsimile image data in the form of audio tones which can be transmitted over voice radios; which provide user-adjustable audio output levels, for compatibility with a wide range of existing and future voice radio equipment without requiring modification to such radios; which provide a connection point through which such audio output signals can be connected directly into the voice microphone input to a radio; which provide a push-to-talk signal to simulate the action of a user-activated talk button on a voice microphone, which signal is connected to the radio, and is activated to switch the radio from listen to talk whenever appropriate in order to transmit facsimile images; which provide a connection point that receives audio input signals from the speaker or earphone output of a radio; which provide automatic gain control to allow operation over a wide range of radio voice output levels; and which provide demodulation capabilities to convert audio tones back into image data.